Multiscale Finite Element Analysis of Unbonded Flexible Risers

Abstract

Unbonded flexible risers are a key technology in existing and proposed offshore developments. With increasing water depth, the demands on risers increase and the design against hydrostatic and tension loads becomes more of a challenge. In addition, many existing subsea production systems are approaching the end of their design life and operators need to know if they can remain in-service. To enable the benefits from deepwater production and life extension projects to be realized while minimizing risks to life, property and the environment, accurate modelling and analysis tools are required to improve the prediction of failure modes and to develop a better understanding of the conditions leading to progressive failure. In this work, a multi-scale approach is adopted whereby a global dynamic analysis model is employed to determine the overall displacements of the riser and this is linked with a local model that can provide accurate forces and stresses for the prediction of collapse, fatigue damage and buckling of tensile armour wires. Firstly, we describe a nonlinear constitutive model for use in large-scale dynamic analysis of flexible risers based on an analytical homogenization of composite cylinders using the analogy between slip between pipe layers and plastic flow in continua. The model is able to reproduce the bending hysteresis behaviour observed in flexible pipes and its dependence on internal and external pressure. Secondly, we show a procedure for obtaining equivalent material parameters for this model from finite element local analyses of a flexible pipe. Finally, we show the implementation of this constitutive model in a riser system using two-dimensional co-rotational hybrid beam finite elements.

Review of the Development of an Unbonded Flexible Riser: New Material, Types of Layers, and Cross-Sectional Mechanical Properties

“…When the material of the unbonded flexible riser changes only in the elastic range, the strain–load curve changes linearly when it is only subjected to axisymmetric loading [ 6 , 7 ]. Under a bending moment, the tendons within helical layers (mainly referring to the tensile armor layer) undergo relative slipping when the bending moment increases, which makes the bending stiffness of the unbonded flexible riser decrease sharply and then the hysteresis phenomenon occurs [ 39 , 40 , 41 , 42 , 43 ].…”

A review on design, manufacture and mechanics of composite risers

“…Though it is proposed as a robust implementation environmental loads and fatigue were ignored making it unsuitable for detailed studies. Recently, Pham et al (Pham et al, 2014) and Edmans et al (Edmans et al, 2014) have described a sequential local-global approach for multiscale modeling of unbonded flexible risers subjected to combined loadings. RVE analyses of flexible pipes were first performed to derive nonlinear constitutive model for flexible risers based on homogenization theory and the analogy between slipping mechanisms observed in flexible pipe and the elasto-plastic behavior of materials.…”

An Effective Multiscale Methodology for the Analysis of Marine Flexible Risers

“…This approach requires bending-moment curvature data to be provided so that nonlinearity in pipe response can be incorporated. Earlier development on the multiscale methodology presented in the current work was detailed by Edmans et al [19][20][21][22].…”